feat(us-027/us-028): throughput-optimized routing + correctness tests

_select_route now prefers nodes with higher effective throughput
(benchmark_tokens_per_sec / (queue_depth + 1)) when multiple nodes
cover the same interval, breaking the tie after max-reach selection.
TorchNodeServer.route_timeout property added for external inspection.

Tests added:
- _select_route with overlapping shards (A:0-22, B:20-24) → correct X-Meshnet-Start-Layer protocol
- _select_route with gap → clear error message
- Fast node beats slow node when shards are equal
- Busy fast node vs idle slow node (queue_depth factor)
- Two-StubNodeServer integration via TrackerServer
- route_timeout property exposed on TorchNodeServer

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/node/meshnet_node/torch_server.py

@@ -628,6 +628,10 @@ class TorchNodeServer:
         self._thread: threading.Thread | None = None
         self.port: int | None = None
 
+    @property
+    def route_timeout(self) -> float:
+        return self._route_timeout
+
     @property
     def backend(self) -> TorchModelShard:
         return self._backend